In short
- WMM is wet mix macadam: crushed, graded aggregate mixed with water in a plant and laid by paver as the base course, under MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), Clause 406.
- The grading is a single envelope in Table 400-13, from 100 percent passing 53 mm down to 0 to 5 percent passing 75 micron.
- Table 400-12 limits the stone: Los Angeles abrasion 40% maximum or impact value 30% maximum, and combined flakiness and elongation 35% maximum.
- A single compacted layer is not less than 75 mm, and up to 200 mm with vibrating compaction and the Engineer's approval. Density is at least 98% of MDD to IS 2720 Part 8.
- The usual failures are segregation, moisture lost on the haul, a fine fraction that is too dusty or too plastic, and flaky stone from the crusher.
Wet mix macadam is the layer the bituminous courses sit on, so it is the layer that shows first when a road starts to fail. It is also the first layer where the specification stops being forgiving: one grading, a limit on stone shape, and a prescribed way of mixing and laying.
WMM is clean, crushed, graded aggregate, mixed with a controlled amount of water in a mixing plant, laid by a paver on the granular sub-base and compacted to at least 98% of maximum dry density. Clause 406 of the MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), governs it. The grading is in Table 400-13 and the physical requirements in Table 400-12.
Where WMM sits and what it has to do
It is the granular base course, laid over the GSB and directly under the dense bituminous macadam. The sub-base below drains and spreads load. The base carries it through stone-on-stone contact, which is why the clause cares how the particles are shaped and packed. GSB against WMM sets the two side by side.
The aggregate is crushed stone. If crushed gravel or shingle is used, Clause 406 wants at least 90 percent of the pieces retained on the 4.75 mm sieve to have two or more fractured faces. Rounded faces do not lock.
The grading in Table 400-13
| IS sieve | Percent by weight passing |
|---|---|
| 53.00 mm | 100 |
| 45.00 mm | 95-100 |
| 26.50 mm | No limit printed |
| 22.40 mm | 60-80 |
| 11.20 mm | 40-60 |
| 4.75 mm | 25-40 |
| 2.36 mm | 15-30 |
| 600 micron | 8-22 |
| 75 micron | 0-5 |
The 26.50 mm row appears in the table without a limit against it.
Nothing is allowed above 53 mm, and no more than 5 percent may stay on the 45 mm sieve. Below that the band is 20 points wide or less on every sieve. A blend can sit comfortably inside at 22.4 mm and still be out at 4.75 mm, the sieve where the coarse fractions end and the dust begins. How to read a sieve analysis works through a result against this table.
Physical requirements in Table 400-12
| Property | Requirement |
|---|---|
| Los Angeles abrasion value, or | 40% maximum |
| Aggregate impact value | 30% maximum |
| Combined flakiness and elongation indices | 35% maximum |
| Plasticity index of material finer than 425 micron | Not more than 6 |
| If water absorption is above 2% | Soundness test to IS 2386 Part 5 |
Abrasion and impact are alternatives: the aggregate has to satisfy one of the two. The last two rows come from the text of Clause 406, not from the table.
The 35 percent limit is on the combined index. Flaky pieces are measured and removed first, elongation is measured on what is left, and the two percentages are added. A crusher can hold the grading perfectly and still fail here. Flakiness and elongation index covers the test and what the cone setting has to do with it.
Mixing, laying and compaction
WMM is not mixed on the road. Clause 406 calls for an approved mixing plant with controlled addition of water and forced or positive mixing, which in practice means a pugmill or a pan mixer. The water is there to bring the mix to the moisture at which it compacts best, the optimum found in the IS 2720 Part 8 test, and to hold the fines to the stone so that the mix does not separate on the way.
Laying is by paver finisher. A motor grader is allowed only in exceptional cases and with prior approval. The limits on the layer are few:
- A single compacted layer is not less than 75 mm.
- It may go up to 200 mm where vibrating compaction is used and the Engineer approves.
- Density is at least 98% of maximum dry density by IS 2720 Part 8.
- Finished surface level is within ±10 mm when machine laid, ±15 mm when laid manually (Table 900-1).
Test frequencies
| Test | Minimum frequency |
|---|---|
| Aggregate impact value | One per 1,000 m3 of aggregate |
| Grading | One per 200 m3 of aggregate |
| Combined flakiness and elongation | One per 500 m3 of aggregate |
| Atterberg limits of fraction passing 425 micron | One per 200 m3 of aggregate |
| Density of compacted layer | One set of three tests per 1,000 m2 |
These are minimums. Standards are revised and contracts add to them, so the frequency in your quality plan is the one that binds.
How WMM fails
- Segregation. Coarse stone runs to the outside of a stockpile, a tipper body and a paver hopper. The laid mat then has bony patches that will not reach density and rich patches that pump fines. A mix that left the plant too dry segregates far more readily.
- Moisture lost in transit. A load that leaves at the right moisture and spends two hours uncovered in May arrives short of it. It rolls, but it does not reach 98 percent. Cover the loads and plan the lead.
- Wrong fines. Too much passing 75 micron, or a plasticity index over 6 on the fraction below 425 micron, usually traces to weathered rock or overburden in the feed. The grading can look acceptable while the fines are quietly clay.
- Flaky aggregate. A cone run starved, a worn liner or the wrong closed side setting turns out slabs. They break under the roller, and the grading in the road is no longer the grading that was tested.
What the crusher has to deliver
No single product off a screen deck is WMM. The mixing plant draws from separate bins, typically a coarse fraction, 20mm, 10mm and dust, and the proportion from each is set by sieving every bin and working out the combination that lands in the middle of Table 400-13. What the mixing plant needs from the crusher is fractions that are the same today as they were when the proportions were set.
That is plant discipline more than specification: mesh replaced before it wears oversize, decks kept from blinding, cone liners changed on schedule, feed kept clean of overburden. Our plants run a primary jaw, a secondary cone and a tertiary stage ahead of the screens, and what each crushing stage does explains why shape is settled in the later stages. The gradings we set the decks to are under stone crushing and aggregate production, and the other sizes off the same plants are on the materials page.
Standards and sources
- MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), Section 400, Clause 406, Tables 400-12 and 400-13
- MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), Section 900, Tables 900-1 and 900-3
- IS 2720 (Part 8): 1983, heavy compaction; IS 2386 (Part 1): 1963, flakiness and elongation; IS 2386 (Part 5), soundness
- Plant configuration: SM Infra's own operating record
Your contract and its technical schedules override anything written here. Check the clause before you build to it.